1. Field of the Invention
This invention relates to C.sub.6 paraffin isomerization in which a preflash tower is used to initially separate liquid product from the isomerization zone.
2. General Background
Paraffin isomerization processes are generally used in modern refineries to enhance the octane value of paraffinic and cycloparaffinic hydrocarbons having from 4 to about 7 carbon atoms. The product from the isomerization zone is normally fed to a fractionation section where selected C.sub.5 and C.sub.6 paraffins and cycloparaffins which have increased octane are concentrated and sent to the refinery gasoline pool. The less desirable or low octane liquids are generally recycled to the isomerization zone. A small portion of the heaviest materials produced in the isomerization zone is generally removed from the process as a slip stream, and light gaseous materials are sent to the refinery fuel gas system.
The typical fractionation scheme for an isomerization zone includes a high pressure separation zone which separates the effluent from the isomerization zone into light gases and an isomerization zone liquid effluent. The latter is recovered and sent to a fractionation section.
In cases where there is no recycle to the isomerization zone (once through operations) the liquid effluent from the separation zone is passed into a stabilizer tower which separates fuel gas from liquid isomerization zone product which has enhanced octane value and can be used in the gasoline pool.
In many isomerization operations, some of the low octane effluent from the isomerization zone is separated from valuable isomerization zone product and recycled to the isomerization zone for additional conversion. These operations are generally known as recycle operations. The fractionation section design in these conventional operations can vary, but in most instances when a new isomerization unit is being built, the fractionation section will comprise two separate towers.
In conventional two tower recycle operations in which no preflash tower is used (see FIG. 2), the first tower is a stabilizer tower into which all the liquid effluent from the separation zone is passed and separated into fuel gas comprising C.sub.3 and lighter hydrocarbons and a stabilizer tower bottoms stream which contains essentially all of the remaining liquid effluent from the separation zone. The liquid feed rate to the stabilizer tower in the above operation is much larger than the liquid feed rate to this tower when operated in once-through operations, since recycle material is contained in feed.
The bottoms stream from the stabilizer tower is then fed into a de-isohexanizer tower for separation into the following three streams: the lightest stream is generally the isomerization zone product having enhanced octane value which is recovered from the overhead section of the de-isohexanizer tower and sent to the refinery gasoline pool; a side stream is drawn off comprising recycle material which comprises C.sub.6 and some C.sub.7 hydrocarbons; and a bottoms stream generally comprising the heaviest components produced in the isomerization zone is generally purged from the system to avoid buildup of these heavier materials.
When a refiner designs a grass-root paraffinic isomerization zone for once-through operations or recycle operations, he has much flexibility in reactor designs along with the sizing of the various towers used in the fractionation section of the isomerization zone process.
In instances in which the refiner desires to alternately operate an isomerization zone in either the once-through or recycle modes, various compromises must be made in the fractionation section in order to provide adequate sizing of both stabilizer and de-isohexanizer towers for these two different operations. In the once-through operation, since there is no recycle of unreacted material to the isomerization zone reactor the stabilizer tower will receive a lower feed rate of liquid effluent when compared to the feed rate of liquid effluent it would receive during recycle operation. In many instances these compromises will result in less than ideal separations or design of tower reboilers or heat exchangers. Costs will not be optimized because the stabilizer tower must be designed for a wide range of flow rates and operating conditions.
The present invention offers an improvement to the refiner having a paraffin isomerization process designed to operate alternately in either the once-through or recycle mode, and which is either being built as a grass-roots project or is a revamp using some existing equipment. In the present invention a process improvement results from the use of a preflash tower during recycle operations to make a gross separation of the separation zone liquid effluent into preflash overhead fraction which contains substantial quantities of isomerization zone product and a preflash bottoms fraction. The latter fraction contains much of the unconverted C.sub.5, C.sub.6 and C.sub.7 materials which boil at higher temperatures than the isomerization zone product, some isomerization zone product and a heavy stream comprising C.sub.6 + materials. This stream is passed into the de-isohexanizer tower for further separation into three major streams.
The three streams are a de-isohexanizer tower overhead fraction which contains concentrated quantities of isomerization zone product which were not separated from the preflash tower bottoms fraction, a middle boiling stream comprising C.sub.5 and C.sub.6 recycle materials which are returned to the isomerization zone for further conversion, and a heavy stream comprising C.sub.6 + materials which are removed from the process to avoid buildup within the processing loop.
The de-isohexanizer zone overhead fraction is combined with the preflash overhead fraction and passed into the stabilization tower. These two streams having been initially sent to the preflash tower now have a reduced quantity of heavier materials resulting in a lower quantity of feed material for the stabilizer tower to process. In the stabilization tower a separation of fuel gas from isomerization product takes place.
By using the preflash tower during recycle operations, the feed rates to both the stabilization tower and the de-isohexanizer tower are reduced since the preflash tower performs an initial liquid-liquid separation on the separation zone liquid. The stabilizer tower, can therefore, be designed for lower feed rates.
The reduced size of the stabilizer tower still allows its use in once-through operations, since in once-through operations a reduced quantity of liquid effluent from the isomerization zone is passed into the fractionation section. In once-through operations, the liquid effluent from the separation zone passes directly to the stabilizer tower for separation into fuel gas and isomerization zone product.